 Dear students, in this topic we shall discuss the role of calcium in contraction. The calcium ions play a crucial role in regulating the contractile activity of the muscle. Calcium amelitore ke upar kya karti hai, it helps to expose the active sites of actin filament which bind the myosin heads. As a result, cross bridges are formed which are necessary for contraction. Calcium ions induce contraction with the help of two regulatory proteins which are associated with the actin thin filament. These proteins are troponin, and tropo-myosin. In a relaxed myofibril, tropo-myosin coils around the actin filaments. During this coiling, the myosin binding sites of the actin are physically covered. As a result, the actin and myosin stop interacting. In addition, the troponin is attached to every 40 nanometer distance with the tropo-myosin. The troponin has high affinity for calcium ions. Every tropo-myosin complex can bind 4 calcium ions with it. As a result, troponin undergoes a change in its conformation. This conformational change in troponin causes a shifting in the position of tropo-myosin. When tropo-myosin moves from its place, the thin filaments of the actin expose the myosin binding sites. The calcium binding to troponin removes that inhibition that was present for binding of myosin with actin. When this inhibition is over, the myosin heads are attached to the actin and cross-bridges are formed. When cross-bridges are formed, the thin filaments of the actin slide past as a result. The muscle fiber is able to contract. Dear students, for the role of calcium ions, it is necessary that the concentration of calcium ions in the myoplasm of the cell is more than 10 raised to the power minus 7 molar.